Fracture of long bones initiates a complex series of cellular events that results in restoration of normal bone anatomy and function. Increasing evidence indicates that these events are regulated by growth factor members of the TGF-B super family. These function via their cognate receptors which are serine/threonine kinases (s/t kinase). Despite the significance of these growth factors in bone and cartilage, however, no s/t kinase receptors have been identified in these systems. This laboratory has recently identified five novel s/t kinase-like genes in bone and callus. To improve understanding of the mechanism of action of these novel and related receptors with respect to fracture repair, this proposal tests the following hypotheses: (1) These novel genes are receptors for the TGF-B super family of proteins. This hypothesis will be tested by complete cDNA sequencing, heterologous expression and identification of cognate ligands for the novel genes. Testing this hypothesis will contribute greatly to the understanding of the growth- factor-receptor interactions that are vital to bone remodeling and fracture repair. (2) Receptors to the TGF-B superfamily of proteins are expressed by specific cells during bone formation, remodeling and repair. This hypothesis predicts that such receptors will be found in the callus dung fracture repair and in the skeleton during bone and cartilage formation. Determination of cellular localization of these receptors should provide clues to the mechanism by which bone remodeling takes place. (3) The slt kinase receptors regulate multiple processes in osteoblasts and chondrocytes by influencing expression of bone and cartilage-related genes. TGF-B has been shown to regulate expression of type II collagen in fracture. repair. This hypothesis will be tested by quantitatively evaluating parameters of osteoblast and chondrocyte function in relation to the expression of these receptors. (4) The expression of these receptors and/or their cognate ligands is altered in the streptozotocin-induced diabetic (STZ-diabetic) model of impaired fracture healing. Preliminary data from this laboratory suggest that altered TGF-B activity is associated with the pathologic changes seen during fracture repair in this model. This hypothesis predicts that ligand receptor interactions in addition to that of TGF-B will be contributory to the mechanism by which the clinical symptom is manifested in these mice. These studies should improve understanding of the mechanism of fracture repair at the molecular level and will provide valuable information on the metabolic processes responsible for impaired fracture healing.